1. Field of the Invention
The present invention relates to a method and an apparatus for polishing a substrate after a back surface of the substrate is grinded, and more particularly to a method and an apparatus for polishing an angular portion formed by the grinded back surface and a circumferential surface of the substrate.
2. Description of the Related Art
In fabrication processes of SOI (Silicon on Insulator) substrate, through-silicon via (TSV), power device (i.e., semiconductor element for electric power), and the like, a back surface of a substrate is grinded for making the substrate thinner. In this grinding process, a grinding tool, which is called a back grinder, is used. The back surface of the substrate is grinded until a thickness of the substrate is reduced to, for example, 300 μm or less. Specifically, the back surface of the substrate is pressed against the rotating back grinder, so that the back surface of the substrate is grinded until the thickness of the substrate reaches a desired thickness.
Generally, a circumferential surface of the substrate is polished in advance in a rounded shape in order to prevent cracking or chipping thereof. When the back surface of the substrate having such a circumferential surface with a rounded shape is grinded, an angular edge is formed on the substrate as a result of the grinding. FIG. 1 shows an example of an acute angular edge as a result of a process of grinding the back surface until the thickness of the substrate is reduced to half or less. This angular edge (which will be hereinafter referred to as angular portion) is constituted by the grinded back surface and the circumferential surface of the substrate. Such acute angular portion is likely to be chipped by a physical contact, and the substrate itself may be broken during transportation of the substrate. Particularly, in the nature of the substrate, once the substrate is chipped, the crack tends to extend into a region where devices are formed, thus causing defects of products. Further, the angular portion may hinder uniform grinding of the back surface, and may even cause cracking of the substrate during grinding.
Thus, in order to prevent such damages of the substrate, it has been customary to remove the angular portion formed on the substrate by bringing a grinding stone (or a lapping tool) into contact with the substrate. More specifically, the substrate is held by a support stage by a vacuum suction or the like with its grinded surface facing upward, and the substrate is rotated about its center. In this state, the grinding stone is brought into contact with the angular portion of the substrate to thereby polish the angular portion.
However, the substrate is not perfectly round and moreover it is difficult to strictly align the center of the substrate with a rotational axis of the support stage. Therefore, when the grinding stone contacts the angular portion of the substrate, the grinding stone may damage the substrate. In order to prevent such damage to the substrate, it is necessary to bring the grinding stone closer to the substrate slowly. However, this results in a longer time for bringing the grinding stone into contact with the substrate (this time is referred to as “air-cutting time”), thus lowering throughput. Moreover, even after the grinding stone contacts the substrate, it is necessary to move the grinding stone very slowly until the angular portion in its entirety is removed by the grinding stone. Consequently, a long grinding time is needed for softening an impact between the grinding stone, which is a rigid body, and the substrate.
Generally, a protection film is attached to the substrate whose back surface is grinded, as shown in FIG. 2. This protection film is for the purpose of protecting devices formed on a front surface (i.e., a surface opposite to the back surface) of the substrate. The protection film is attached so as to cover the front surface and the circumferential surface of the substrate. Polishing of the angular portion of the substrate with the protection film attached is performed basically in the same manner as described above. Specifically, the front surface of the substrate with the protection film attached thereto is held by the support stage, and the angular portion of the substrate, rotated by the support stage, is polished by the grinding stone. The grinding stone polishes the angular portion while grinding the protection film.
However, if an adhesive, which is used for the protection film, exists unevenly in a circumferential direction of the substrate, the protection film may locally hinder polishing of the substrate. As a result, the substrate is unevenly polished. Further, the adhesive may be attached to the grinding stone, thus lowering the polishing performance of the grinding stone.